In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. On the transmit side of a transceiver module, a light source (e.g., a laser diode) generates amplitude modulated optical signals that represent data, which are received by an optics system of the transceiver module and focused by the optics system into an end of a transmit optical fiber. The signals are then transmitted over the transmit fiber to a receiver node of the network. On the receive side of the transceiver module, the optics system of the transceiver module receives optical signals output from an end of a receive optical fiber and focuses the optical signals onto an optical detector (e.g., a photodiode), which converts the optical energy into electrical energy.
The transmit and receive fiber cables have connectors on their ends, often LC connectors, that are adapted to mate with transmit and receive receptacles, respectively, formed in the transceiver module. A variety of optical transceiver module configurations are used in optical communications network. Some optical transceiver modules have multiple transmit receptacles and multiple receive receptacles for connecting multiple receive and transmit fiber cables to the module. Some transceiver modules having a single receive receptacle and a single transmit receptacle arranged side by side for connecting a single receive fiber cable and a single transmit fiber cable, respectively, to the transceiver module.
The transceiver modules themselves typically also have mating elements on them that are adapted to mate with mating elements formed on the cages into which the modules are inserted. The cages are typically contained in racks, and each rack typically includes many cages that are arranged in very close proximity to one another. Each of these cages is configured to receive a transceiver module on the front side of the rack through a front panel of the rack. The transceiver modules are configured so that they may be inserted into and removed from the cages. The modules typically include latching mechanisms that couple to mating features on the cages when the modules are inserted into the cages. In order to remove a module from a cage, the module must be de-latched to decouple the latching mechanism from the features on the cage, which can be challenging when the modules are spaced closely together in the racks.
Most optical transceiver modules include one or more electromagnetic interference (EMI) sealing components that are designed to prevent electromagnetic radiation from escaping from the transceiver modules. The Federal Communications Commission provides standards that limit the amount of electromagnetic radiation that may emanate from unintended sources. A variety of techniques and designs are used to shield potential EMI openings in optical transceiver modules in order to limit the amount of EMI radiation that may pass through the openings and thereby propagate into the environment outside of the modules. One area in transceiver modules that constitutes a large EMI opening is the backside of the transceiver module opposite the printed circuit board (PCB). The PCB typically contains electrical components that are EMI sources, such as the laser diode driver integrated circuit (IC), which drives the laser diode of the transmit side of the transceiver module. Often times, an EMI absorbing assembly comprising a sheet of EMI absorbing material, such as a sheet of Eccosorb® material or the like, is placed in the transceiver module housing in a cavity between the backside of the module housing and the laser diode driver IC to prevent EMI generated by the laser diode driver IC from passing through the backside of the module housing. Eccosorb® material is an electromagnetic radiation absorbing material manufactured by a company called Emerson & Cuming Microwave Products, Inc. One of the problems associated with using a sheet of absorbing material disposed in free space in the housing is that it absorbs the magnetic, but not the electrical, components of the EMI. Therefore, the absorbing assembly is not a completely effective EMI sealing solution when employed in this manner.
Another problem associated with using an EMI absorption assembly comprising a sheet of radiation absorbing material disposed in free space in the housing is that such a solution does not prevent resonant modes of the fundamental frequency of the laser diode driver IC from occurring in the module housing. The length of the cavity in the module housing in which the sheet of material is placed often has a length that can allow resonant frequencies of the fundamental frequency to occur in the housing. These resonant EMI modes also need to be sealed within the housing in order to be compliant with EMI sealing requirements.
Accordingly, a need exists for an EMI absorbing assembly that is effective at shielding EMI in an optical transceiver module. A need also exists for an EMI absorbing assembly that prevents or lessens the occurrence of resonant frequency modes in the transceiver module that also must be dealt with by the EMI sealing solution used in the module.